Holographic projection devices modulate sufficiently coherent light by means of spatial light modulator means. Due to interference of the light, object light points reconstructing the optical appearance of a scene are created in a space in front of, on and behind the surface of the light modulator. The entirety of the light of all object light points propagates in the form of a light wave front, so that one or multiple observers observe those object light points in the form of a three-dimensional scene. This means that in contrast to a stereoscopic representation, a holographic reconstruction realises an object substitute, and the problems known in conjunction with stereoscopy, such as fatigue or aching eyes and headache, do not occur, because in principle there is no difference between the observation of a real scene and a holographically reconstructed scene.
From the earlier patent application DE 10 2005 023 743 filed by the applicant a holographic projection system is known, where a pupil is situated in a Fourier plane of a spatial light modulator (SLM) and acts as a spatial filter, for a diffraction order of the Fourier transform of a hologram provided by the SLM. This pupil is projected by a deflection element or adaptive mirror into a visibility region in an observer plane; from the visibility region an observer can watch an enlarged holographic reconstruction of a real existing object or a scene. In other words, the visibility region is the image of the diffraction order used e.g. of the Fourier transform (the Fresnel transform etc. would be possible as well) of the hologram in the observer plane, i.e. the plane where the eye position of the observer is situated. The size of the adaptive mirror determines the size of the reconstruction. For example, the adaptive mirror preferably has a size of about 20 inches diagonal.
At the same time it must be noted that the larger the visibility region the higher must be the resolution of the SLM used. In order to get a large visibility region, the SLM must have small pixel apertures which cause a large diffraction angle, i.e. the pixel pitch must be small and, consequently, the number of pixels must be large.
In order to reduce the necessary resolution of the SLM, the size of the visibility region can be diminished for example to the size of an eye pupil. It is therefore necessary for the visibility region to be tracked to the observer eyes if the observer moves. The adaptive mirror must project the pupil into the visibility region.
So called adaptive MEMS (micro electro-mechanical system) mirrors are known from the prior art. These comprise a micro-mirror array, the micro-mirrors being capable of performing tilting and lifting movements. This allows creating a surface which has any curvature within a given adjustment range. However, until now such micro-mirror arrays were only available in sizes up to about 1 inch diagonal. An adaptive mirror with a size of, for example, about 20 inches diagonal, as required in a holographic projection system, would be very difficult to make, and it would require a very large number of movable mirrors.
An electronically addressable spatial light modulator (EASLM) as a controllable diffractive optical element (DOE) would have a very large number of small pixels because of the large size and the required large diffraction angles, and it is thus not feasible technologically to realize such an element. Assuming a pixel pitch for example of 5 μm, a SLM with a diagonal of 20 inches would comprise 5*109 (five billion) pixels. This would be about three decimal powers (one thousand times) more pixels than EASLMs commercially available today have.